This invention relates to the treatment of superphosphoric acid and more particularly refers to an improved process for removing magnesium from wet process superphosphoric acid.
Wet process phosphoric acid is conventionally prepared by reacting sulfuric acid and phosphate rock, followed by filtration to remove the insoluble gypsum and other insoluble compounds. The resultant dilute or weak phosphoric acid containing about 26 to 30 percent P.sub.2 O.sub.5 by weight is commonly known as "filter" acid and is a highly impure material containing dissolved sulfates, fluosilicates, and salts of iron, aluminum, magnesium, sodium and other metals. These impurities may precipitate and settle out in varying rates and amounts during storage or further processing of the dilute wet process phosphoric acid.
Concentration of dilute or weak wet-process phosphoric acid up to the super range containing 64 to 72 percent P.sub.2 O.sub.5 is done in two steps. Preferably this two step concentration is done in separate equipment because of variation in temperature, corrosion and viscosity that occur through the total range.
As a first step, it is common to evaporate said dilute or weak acid and to partially purify said acid by removal of precipitated impurities consisting of CaSO.sub.4, Na.sub.2 SiF.sub.6, (Fe, Al).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O and other salts to a concentration of about 38 to about 56 weight percent P.sub.2 O.sub.5. This acid is known as "evaporator" acid with about 54 percent P.sub.2 O.sub.5 being most common. It is difficult to remove magnesium at this stage due to the high solubility of magnesium salts.
As a second step, the partially purified evaporated acid (38 to 56 weight percent P.sub.2 O.sub.5) is further evaporated to superphosphoric acid containing about 64 to 72 weight percent P.sub.2 O.sub.5. Impurities that precipitate in the production of the superphosphoric acid consist of MgH.sub.2 P.sub.2 O.sub.7, FeH.sub.2 P.sub.3 O.sub.10, AlH.sub.2 P.sub.3 O.sub.10 and other salts. Filtration of superphosphoric acid removes a portion of the magnesium, iron, and aluminum impurities, but such filtration of the superphosphoric acid is a difficult and slow process. The filtration rate is very slow, due to the high viscosity of the superphosphoric acid and the small crystals that have dimensions in the range of 1 to 15 microns.
Liquid ammonium phosphate fertilizer solutions are derived from wet process phosphoric acid. Said solutions, commonly 10-34-0 grade (10 weight percent N, 34 weight percent P.sub.2 O.sub.5, and 0 weight percent K.sub.2 O), are prepared either (1) by reacting superphosphoric acid containing 64-72 percent P.sub.2 O.sub.5 with liquid and/or gaseous ammonia or (2) by reacting acid containing 54 to 60 weight percent P.sub.2 O.sub.5 with gaseous ammonia. Magnesium is a particularly troublesome impurity in such prepared liquid ammonium fertilizer solution because it slowly precipitates as Mg (NH.sub.4).sub.2 P.sub.2 O.sub.7.4H.sub.2 O or as MgNH.sub.4 PO.sub.4.6H.sub.2 O. The settling of these precipitates results in sludge losses in storage tanks or plugging of handling equipment.
Various prior art methods have previously been proposed for limiting precipitation of the magnesium salts or for removing magnesium from phosphates.
One method, described in U.S. Pat. No. 3,632,329, teaches a method for preventing post-precipitation of magnesium salt from ammonium phosphate fertilizer base solutions prepared from wet superphosphoric acid by the accelerated precipitation of Mg (NH.sub.4).sub.2 P.sub.2 O.sub.7.4H.sub.2 O. The method comprises continuous agitation of said solution concurrently with seeding at specified temperatures and pH followed by separation of the precipitated magnesium sludge.
A second method, described in U.S. Pat. No. 3,554,728, teaches the accelerated precipitation of Mg(NH.sub.4).sub.2 P.sub.2 O.sub.7.4H.sub.2 O in said solutions by means of overammoniation to high N/P.sub.2 O.sub.5 ratio followed by separation of the sludge and adjustment back to the desired N/P.sub.2 O.sub.5 ratio.
The disadvantage of these methods is that disposal or by-product use of the magnesium sludge containing valuable fertilizer P.sub.2 O.sub.5 and N can be very costly.
A third method, described in U.S. Pat. No. 3,642,439, involves forming a precipitate of a magnesium-aluminum-fluoride-phosphate complex compound from phosphoric acid. The process involves the following steps:
(a) evaporating the weak phosphoric acid at a temperature of 85.degree.-100.degree. C. at a pressure below atmospheric to a concentration of 45-53 weight percent P.sub.2 O.sub.5, preferably 47-51 weight percent P.sub.2 O.sub.5, whereby the H.sub.2 SiF.sub.6 content of the acid is reduced and the hydrogen fluoride content is increased;
(b) maintaining the hydrogen fluoride content of the concentrated phosphoric acid at F/MgO weight ratio of at least 2.2, preferably between 3 and 12;
(c) maintaining the soluble aluminum content of the concentrated phosphoric acid, measured as Al.sub.2 O.sub.3, at an Al.sub.2 O.sub.3 /MgO weight ratio of at least 1.4, preferably between about 3 and 12;
(d) maintaining the concentrated phosphoric acid at 50.degree.-100.degree. C. for 15-40 hours to form a precipitate comprising a crystalline filterable magnesium-aluminum-fluoride-phosphate complex compound; and
(e) separating the precipitate from the purified concentrated phosphoric acid. The additives required for this process are costly.
A fourth method, described in U.S. Pat. No. 3,711,268, relates to adding a soluble fluorine compound to ammoniated superphosphoric acid to partially precipitate the magnesium impurities or partly stabilize the liquid. This method has the drawback that the fluoride reagent is very costly. A similar method utilizes less costly 23 percent by weight H.sub.2 SiF.sub.6 to precipitate MgSiF.sub.6.6H.sub.2 O from phosphoric acid, but costs are still relatively high considering dilution cost and cost of H.sub.2 SiF.sub.6.
Another method involves pretreating phosphate ore prior to conventional sulfuric acid leaching by leaching with an acidic solution to dissolve the magnesium impurities. But many phosphate rocks cannot be leached to a sufficiently low magnesium content.
The process of the present invention overcomes the shortcomings of prior art processes for removing magnesium from superphosphoric acid.